1. Field of the Invention
The present invention relates to lubricating oil compositions for use with bearing assemblies that use sintered porous bearings. More particularly, the invention relates to lubricating oils suitable for impregnation in sintered porous bearings that are typically used in small motors, etc.
2. Related Art
Sintered, oil-filled bearings for use in high-speed and light-load applications have the advantage that they can be operated without additional oil supplies during service. Because of this feature, sintered, oil-filled bearings are extensively used in bearing assemblies for a variety of small motors such as motors to rotationally drive magnetic disks and motors to operates audio-related instruments and various other machines and equipment.
With the recent advances in the performance of the machines and equipment with which the sintered, oil-filled bearings are used, increasingly high and versatile performance has been required of those bearings and this has given rise to the need for a sophisticated lubrication technology.
There have been two approaches in the improvement of bearings, one by improving the properties of the metals to be sintered and the other by improving the lubricating oils to be impregnated in the sintered metals.
The increasing tendency in the art is putting emphasis on the characteristics of lubricating oils. This is chiefly attributable to the mechanism of lubrication in sintered, oil-filler bearings; although lubricated with oils, these bearings do not operate by fluid-film lubrication but are used in a state that is close to boundary lubrication; therefore, the performance of the bearings will depend largely upon the characteristics of the lubricating oils with which they are filled.
The lubricating oils for use with sintered porous bearings are generally required to have the following characteristics:
(1) permit low current values (hence, small power consumption); PA1 (2) will shortly "break in" and undergo no changes; PA1 (3) can be used form low to high temperatures (-40.degree. to 120.degree. C.); PA1 (4) can withstand high speeds (about 30,000 rpm); and PA1 (5) can withstand low speeds (about 50 to 180 rpm).
The lubricating oils conventionally used with sintered porous bearings are based on various paraffinic and naphthenic mineral oils, as well as ester-based, polyolefinic and various other synthetic oils and these lubricating oils are used in diverse applications as appropriate to their specific characteristics.
A typical example of small motors that use a sintered, oil-filled bearing and that have a bearing holder will now be described with reference to FIG. 1. As shown, a substrate 10 is overlaid with a spacer 12 and a stator core 14. The stator core 14 has a central hole into which a bearing holder 16 is partly fitted and a flange portion 18 molded integrally with the bearing holder 16 is placed on top of the stator 14. A screw penetrating the flange portion 18, stator core 14 and spacer 12 is threaded into the substrate 10, whereby the bearing holder 16, stator core 14 and spacer 12 are fixed to the substrate 10. The stator core 14 has a plurality of salient poles and a drive coil 15 is wound around each salient pole.
Two sintered, oil-filled bearings 22 are pressed against the inner periphery of the bearing holder 16. The sintered, oil-filled bearings 22 compose a radial bearing unit that rotatably supports a shaft 24 inserted through the center of the motor so that it contacts the inner periphery of each bearing 22. As shown, the lower end of the shaft 24 contacts a thrust receptacle 26 filled in a hole in the substrate 10 and the thrust load to be exerted on the shaft 24 will be carried by the receptacle 48. A rotor 28 is coupled to the upper end of the shaft 24 which projects above the upper bearing 22. An annular drive magnet 25 is secured to the rotor 28 and the inner peripheral surface of the magnet 25 is opposite to, but spaced from, the outer peripheral surfaces of the salient poles of the stator core 14.
The rotor 28 and the shaft 24 which is integral with it are rotationally driven by successive on-off control on the supply of an electric current to the drive coils 15 in accordance with the rotating position of the drive magnet 25. The sintered, oil-filled bearings 22 have a very large number of micropores (not shown) which are filled with a conventional lubricating oil. As the shaft 24 rotates, the conventional lubricating oil oozes from the sintered, oil-filled bearings 22 and lubricates the surface of the shaft 24 as it slides against the bearings 22.
As already described hereinabove, the conventional lubricating oils for impregnation in sintered porous bearings have been based on paraffinic or naphthenic mineral oils, as well as ester-based, polyolefinic and various other synthetic oils and these lubricating oils are used as appropriate to their characteristics. No lubricating oils are used exclusively with sintered porous bearings and, instead, suitable types are selected from among commercial hydraulic working oils, engine oils, etc. (see "Gekkan Toraiboroji (Monthly Tribology)", February 1992, p. 60.)
Conventional common lubricating oils have oxidation inhibitors, rust inhibitors, foam inhibitors and metal inactivators added to base oils. In certain cases, other additives are incorporated such as detergent-dispersants, viscosity-index improvers and pour-point depressants.
Lubricating oils based on mineral oils have additional problems in that the paraffin content crystallizes as wax under low temperature to permit the passage of a larger current and that impurities or the products of their reaction with additives will crystallize to form sludge deposits that promote the wear of the rotating shaft and permit the passage of a greater current.